azp: Provide basic auto completion in extension

[ChristopherHX]

e.g. Hook the TemplateReader with the location of the cursor and build a completion item list based on the Schema

Additionally provide snippets for expressions, ${{ if cond }}:, insert, each, else when they are allowed etc.

Providing schema completion like yaml keys and values might make sense nested in if,else,each and insert even if everything could be valid as long this blocks are skipped

When working can be quickly expanded to GitHub Actions.

Another interesting part would be expressions completion.

A toggle to turn this off could be considered